When a defenseman leads the team in scoring, especially goals (35% more than the closest forward), thats a pretty good season. How it translates to the NHL, I have no idea. Just saying he had an impressive season.
No question he had a great season -- no one is disputing that. The relevant question is whether or not it's repeatable. In short, is this his "new normal" and he's just a late bloomer, or was it a fluke?
I don't have nearly enough information to answer that question, but the fact that he really hasn't shown any signs of this level of play before this season is at least a little worrisome.
As for translating his career season to the NHL, the rough estimate based on NHLe is about 0.38 ppg at the NHL level. (This is the NHLe, or NHL equivalency, which Feds referenced, and it's based on previous players who jumped from Finland to the NHL and what happened to their scoring rates after the switch.)
Now, caveats. (1) Every player is different, and NHLe works with averages. So an individual player may end up much better or worse than the average expectation. (2) I suspect, though haven't done enough research to claim, that NHLe might be more accurate for forwards than defensemen.
But if we take that 0.38 ppg as the "best guess" approximation, that would put Kaski around the 60th best defender in the league offensively. So maybe a strong second pairing guy, depending on his defensive aptitude.
And again, that 0.38 ppg also assumes his career year is not a fluke, which it very possibly is.
In short, keep your expectations low. There's a chance he could be a late-blooming solid NHLer, but I think it's more likely we're getting Libor Sulak 2.0. Still, I'm all onboard for a "free" prospect. No risk, high(ish) potential reward.